Long terminal repeat (LTR) containing retrotransposons are mobile elements capable of synthesizing and integrating a DNA copy of their RNA genome into a new location in the host DNA. Many structural and mechanistic features of LTR-retrotransposons are shared with retroviruses. The long-term goal of the proposed research is to understand how the eucaryotic cell regulates the mobility of retrotransposons and retroviruses. The Ty1 retrotransposon of Saccharomyces cerevisiae is used as a model system. Most of the Ty1 elements in the genome are functional for transposition and efficiently transcribed. However, transposition events are rare because post-transcriptional steps in transposition are tightly regulated. Recently, Est2, a component of the reverse transcriptase that synthesizes telomeres, and Te11, a kinase involved in telomere length regulation and a homolog of the human disease gene, ATM, have been identified as inhibitors of Ty1 transposition. These and other findings have led to the hypothesis that Ty1 transposition is regulated by cellular pathways whose normal role is to maintain the integrity of the genome. We propose to systematically identify RTT (regulator of Ty1 transposition) genes in order to characterize cellular pathways that maintain the transpositional dormancy of Ty1 elements. In addition, we will investigate the mechanism of inhibition of transposition by Est2 and Te11. The specific aims of this proposal are to: 1.Perform a genome-wide search for yeast mutants with elevated levels of Ty1 transposition (rttdelta mutants). Prove that the putative rttdelta mutation is the cause of increased Ty1 transposition. 2. Quantify the levels of Ty1 RNA, protein, cDNA and integration upstream of tRNA genes in rttdelta mutants to classify RTT genes. 3. Analyze different steps in Ty1 transposition in es2delta and te11delta mutants to investigate the mechanisms of inhibition of Ty1 transposition by EST2 and Te11. Determine if Est2 and Te11 directly inhibit Ty1 replication or if telomere shortening acts as a signal to derepress Ty1 transposition. 4. Determine if Ty1 transposition or the Ty1 replication machinery can promote protection of chromosome ends in the absence of telomerase. AIDS and adult T-cell leukemia/lymphoma are the consequences of retroviral infection in humans, and cancer and various inherited disorders have been attributed to insertion of retrotransposons. It is essential to understand how the host cell interacts with retroelements because of the potential of retroelements to cause disease. We will determine if a specific type of damage to the genome, loss of telomere, can result in the induction of Ty1 transposition. Loss of normal telomere function is associated with cellular senescence. Hence, the proposed experiments will determine if there is a connection between cellular aging and an inability to maintain the dormancy of retrotransposons or by analogy, the latency of retroviruses.